Loading an image to a texture from a python script file
Blender v2.79
Load image to texture:
bpy.ops.image.open(filepath='full_path_to/your_image.png')
bpy.data.images['your_image.png'].pack(as_png=True) # Pack an image as embedded data into the .blend file
ui_tex.image = bpy.data.images['your_image.png']

Don't forget to replace 'full_path_to/your_image.png' to your own values.
EDIT:
I think this method is easier, you simply load the node, given its name and then set the path:
im_node = scene.node_tree.nodes[&quot;Image&quot;]
im_node.image.filepath = 'full_path_to/your_image.png'